This application is based upon and claims the benefit of priority from the prior Japanese Patent Applications No. 2000-076973, filed Mar. 17, 2000; and No. 2001-009291, filed Jan. 17, 2001, the entire contents of both of which are incorporated herein by reference.
1. Field of the Invention
The present invention relates to a color image generation apparatus and a color image generation method for recording dots of chromatic and achromatic colors and expressing the gradation and forming an image by varying dot sizes of chromatic and achromatic colors.
2. Description of the Related Art
A color image generation apparatus is provided with a plurality of heads for recording chromatic color dots such as C (Cyan), M (Magenta), and Y (Yellow), and achromatic color dots such as K (Black). The color image generation apparatus uses these heads to generate an image by recording chromatic and achromatic color dots on a recording medium.
Unlike monochrome image recording such as character-only recording, the color image generation apparatus recording color images requires various factors such as colorimetric characteristic, tone reproduction characteristic, uniformity of color, and the like.
Especially, the uniformity of color depends on various reasons such as inconsistent nozzle accuracies resulting from a manufacturing process, relative misalignment between recording heads environmental condition changes during recording, and the like.
Ultimately, positional inaccuracy for recording dots degrades the uniformity of color, namely causes a color change.
The color image generation apparatus which expresses intended gradation by varying dot sizes very often generates an image by recording small dots particularly at a highlight portion, namely a region with high brightness. This means that a void region often occurs between adjacent pixels.
The frequency of color changes remarkably increases when recording positions for respective color dots are misaligned for some reason and a different color dot is recorded at the void region.
For example, Jpn. Pat. Appln. KOKAI Publication No. 1-228376 discloses a gradation expression method for suppressing occurrence of such a color change. According to this disclosure, a proper gradation level is specified as a threshold value. The gradation is formed by performing the 100 UCR (under color removal) in a low gradation region below the threshold value. In addition, the pixel gradation is formed by performing the UCR at a decreasing ratio in proportion to a gradation increase in a high gradation region above the threshold value.
This gradation expression method is used for, say, an ink-jet color printer. In this case, the threshold value is set to a gradation level near the boundary where a color change is inconspicuous visually. A region is divided into two portions at the specified threshold used as a boundary.
The 100% UCR is performed in the low gradation region below the specified threshold value. The gradation is expressed with the small number of ink colors, making a color change inconspicuous on a color image. Achromatic color ink such as black is used for forming an image having achromatic color gradation in the low gradation region, providing a color image free from a color change.
When an image is formed in the high gradation region above the specified threshold value, the UCR is performed at a decreasing ratio in proportion to a gradation increase. A color image in the high gradation region yields a final density by repeating the highest one of densities formed by achromatic color ink such as black. Accordingly, such a color image is formed with the smooth, natural tone reproduction characteristic and many gradations.
When the 100% UCR is performed in the low gradation region with a conspicuous color change, the image is apparently formed of the smaller number of colors than usual. However, forming an image with the smaller number of colors than usual does not directly decrease occurrence of color changes. Rather, a combination of chromatic and achromatic colors may cause a more remarkable color change.
No chromatic color is recorded when the 100% UCR is performed in an achromatic color region with the low gradation during image formation. Since only the achromatic color is recorded, no color change occurs, but the granularity degrades.
It is an object of the present invention to decrease such color changes as described above, by using image processing means.
When chromatic and achromatic color dots are recorded at almost the same position, the present invention includes image processing means for converting and reproducing an input color signal and processing the signal so that the sum of dot diameters becomes greater than or equal to a preset specified value.
When chromatic and achromatic color dots are recorded at almost the same position, the present invention includes image processing means for converting and reproducing an input color signal and processing the signal so that the sum of dot areas becomes greater than or equal to a preset specified value.
Additional objects and advantages of the invention will be set forth in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. The objects and advantages of the invention may be realized and obtained by means of the instrumentalities and combinations particularly pointed out hereinafter.